[patch] Add new DW_AT and DW_FORM codes for Fission

[Cary Coutant]

> If the DW_FORM_ values are meant to be in a vendor range of forms
> (which DWARF4 unfortunately doesn't have), aren't they too low, i.e. isn't
> there risk that eventually they'll clash with standard forms?
> The forms numbers aren't limited to 0 .. 0x7f, at the expense of a slightly
> bigger abbrev table you could as well use something very high, like 0x380+.
> And, if you want to standardize it, I'd assume the codes would be lower
> on the other side (there are still even free codes from 0x1a through to
> 0x1f).

FORM codes aren't added very often because they do break
compatibility, so I wouldn't expect values in the 0x70-7f range to
collide with any future standard FORM codes. I did want to keep them
in the single-byte LEB128 range, although as you point out, it
wouldn't cost all that much to make them even larger. We can move them
to a higher range if you prefer.

I do intend to standardize these once we start working on DWARF-5, but
I felt it was a good idea to try them out experimentally before
reserving any standard values.

-cary